Process for producing a polymer which can be used as a powder coating material and/or hot-melt adhesive

ABSTRACT

A process for producing a polymer which can be used as a powder coating material or a hot-melt adhesive in which at least one polyolefin is subjected to a low-temperature plasma treatment with frequencies changing during the treatment in a frequency range of from 30 kHz to 10 GHz.

BACKGROUND OF THE INVENTION

The invention relates to a process for producing a polymer which can beused preferably as a powder coating material and/or hot-melt adhesiveand to its use.

Processes for producing powder coating materials in which generally amixing of polyethylene with α-olefinic polymeric additives takes placeare already known (Catalog MIRATHEN Leuna Werke AG, 10/92). Thepolymeric additives contain certain functional groups which permitcorresponding use of the powder coating material produced therefrom. Itis disadvantageous in this case that there can only ever be produced onepowder coating material for a limited application in each case. On theother hand, it is not possible to produce a powder coating material fora broad field of applications.

Furthermore, hot-melt adhesives are known, in particular on the chemicalbases of polyvinyl acetate, polyamides, ethylene-vinyl acetatecopolymers and polyester. These hot-melt adhesives are likewiseformulated for certain specific applications, the requirements of whichthey meet, so that there are strict limits to any possibility ofversatile use of the hot-melt adhesives.

DE-A 41 41 805 discloses a process and an apparatus for producingthermoplastic parts with the aid of low-temperature plasmas. Thisinvolves treating powdered or granular raw materials withlow-temperature plasma before they are molded into a semi-finishedproduct or component, in order to obtain a modification of the powdergrain surface of the raw materials over a limited period of time.

The invention is therefore based on the object of providing a process ofgeneric type by which powder coating materials and/or hot-melt adhesiveswhich are versatile, that is to say can be used for a multiplicity ofdifferent application areas, can be produced in a simple and inexpensiveway.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by polyolefins beingsubjected to a low-temperature plasma treatment with frequencieschanging during the treatment in a frequency range from 30 kHz to 10GHz. Low-temperature plasma treatment is understood here to mean thatthe material to be treated is itself not heated beyond a certaintemperature range, for example 40° C. to 60° C. It has surprisingly beenfound that, by this plasma treatment, the introduced polyolefins can beadjusted such that they can be used as powder coating materials orhotmelt adhesives generally, in particular also with differentmaterials.

In an advantageous refinement of the invention it is envisaged that theplasma treatment is carried out preferably in a frequency range from13.56 MHz to 2.45 GHz. In particular if the plasma treatment is carriedout at changing frequencies, preferably with combinations of varyinglyhigh, changing frequencies, it is possible very advantageously to caterfor a different chemical structure and different compositions of thepolyolef ins serving as starting materials, so that in each case powdercoating materials or hot-melt adhesives having the properties necessaryfor their intended use can be produced.

In a further preferred refinement of the invention, it is envisaged thatthe plasma treatment is carried out under the supply of an inert gas,for example helium and/or argon, or under the supply of a reaction gas,for example oxygen and/or nitrogen. It is further preferred if theplasma treatment is carried out successively with an inert-gas plasmaand at least one reaction-gas plasma or a reaction-gas plasma mixture orunder the supply of a mixture of at least one inert gas and one reactiongas. By selection of a composition of the process gas matched to thestarting material (inert gas, reaction gas, reaction gas mixture) it ispossible to incorporate to an adequate extent the reactive groupsrequired for the purpose of a coating and/or adhesive bonding, forexample hydroxyl groups, carboxyl groups, primary and secondary aminogroups. These groups are capable of reacting with the material to becoated and/or to be adhesively bonded and of forming chemical bondsand/or of bonding physically. Likewise created polar, but non-reactivegroups (carbonyl groups, tertiary amino groups) effect only a physicalbonding.

In particular in the case of a succession of combinations of differentfrequencies, these combinations being matched to the respective processgas and to the starting material, powder coating materials and/orhot-melt adhesives having desired properties can be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below in an exemplaryembodiment with reference to the associated drawings, in which:

FIG. 1 shows a process sequence for producing a powder coating materialand/or a hot-melt adhesive in a flow diagram and

FIG. 2 shows a schematic representation of an arrangement for carryingout the process.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, the process is intended to be illustrated with the aid of adiagram. In a first step 10, the polyolefins Available as startingmaterials are prepared. The polyolefins are brought here into a powderedand/or granular form, the grain size of which is, for example, less than1 mm, any required additives, such as for example stabilizers, beingadded. Both virgin polyolefins and polyolef ins obtained as recycledmaterial may be used as starting materials. In the case of polyolefinsfrom recycled material, it must be ensured that they are not excessivelydegraded, that is to say that there is still a suitable molecularstructure. In a next step 12, the prepared starting material isintroduced into a process chamber. The process chamber may in this casebe, for example, a rotary drum of a plasma furnace, known per se, forcarrying out a low-temperature plasma treatment.

In a next step 14, the process parameters and process gases desired forthe treatment of the starting material are set. In particular, thespecific combinations of the process gases are fixed, that is to say afirst treatment with an inert-gas plasma, preferably with helium and/orargon, and the following treatment with a reaction-gas plasma,preferably with oxygen and/or nitrogen, or else the treatment with aplasma which is produced from a mixture of the abovementioned gases.Furthermore, the setting of the high frequencies necessary for plasmageneration in a vacuum and their succession in time is performed.

Thus, variants in which a plasma treatment is carried out first ofall-at a low frequency, for example 13.56 MHz, and subsequently at ahigher frequency, for example 2.45 GHz, are conceivable. In addition, analternating cutting in of the frequencies is conceivable. It goeswithout saying that other frequencies can also be set in any desired,freely selectable sequence for carrying out the plasma treatment.Moreover, the desired rotational speed of the rotary drum is set, forexample in the range between 4 and 20 revolutions per minute, and thedesired process pressure, which lies for example in the range between0.3 mbar and 1 mbar. During the plasma treatment, the process pressuremay be subject to fluctuations, process-dependently. Furthermore, theduration of the treatment, over which the treatment of the startingmaterial takes place, is fixed. This is, for example, between 15 and 600s. The said process parameters and process gases may be varied with oneanother in any desired combination and are matched in particular to thecomposition of the respective actual starting material.

In a next step 16, the plasma treatment of the starting material thentakes place with the process parameters and process conditions set instep 14. In this case it is likewise conceivable that, in step 16, achange in and/or adaptation of the process parameters may be performedduring the plasma treatment, for example by a closed-loop control. Theincorporation to an adequate extent into the polyolefins of the reactivegroups required for the purpose of coating and/or adhesive bonding,dependent on the starting material used, can be achieved by thedescribed combination of process gases and process parameters oflow-temperature plasma treatment.

The powder coating materials and/or hot-melt adhesives produced by theprocess according to the invention can be produced from all compoundsbelonging to the group of polyolefins, for example from all polyethylenegrades (LDPE, LLDPE, HDPE) and also polypropylene.

With the powder coating materials and/or hot-melt adhesives produced,all materials of which the melting temperature is above the processingtemperature of the materials according to the invention, for exampleabout 120° C. to 180° C., can be coated and/or adhesively bonded. Thesemay be, for example, such different materials as glass, ceramic, steel,aluminum, wood, paper and polymers. These materials may be precoated orotherwise pretreated. With the materials according to the invention,coatings and/or adhesive bondings which are largely resistant toexternal mechanical and/or chemical influences and have a durable, goodadhesive strength can be achieved. By virtue of these advantageousproperties, use of the coating materials produced by the processaccording to the invention is possible preferably in those areas whichimpose very high requirements on a coating. These are, in particular,underfloor coating in automobile construction or a coating of ships'hulls. In addition, any other possible use is of course conceivable

The construction of an apparatus which can be used for the process isschematically shown in FIG. 2. A low-temperature plasma installation,denoted generally by 22, has a rotary drum 24. The rotary drum 24 servesas process chamber and may consist, for example, of a resistant materialsuch as aluminum or stainless steel. For the creation of a vacuum, theprocess chamber can be hermetically sealed, it not being intended hereto go into any further details. The rotary drum 24 is assigned a device26, which serves as a reactor and is coupled to a generator 28 formicrowave plasma excitation and to a high-frequency feed 32.Furthermore, feed lines 30 for supplying process gases are provided.

The arrangement represented here is only by way of example, and theinvention does not relate specifically to the actual construction of theinstallation. The process according to the invention can of course alsobe carried out by an analogous apparatus which performs the individualprocess steps.

The process according to the invention thus proceeds in the followingway. The selected and prepared starting material is introduced into therotary drum 24 and mixed there in a manner corresponding to the chosenspeed of rotation and direction of rotation, which may also be chosen toalternate. The chosen process gas or process gas mixture is fed via thefeed lines 30 and a plasma is generated in the reactor 26 by means ofthe generator 28. The plasma generation may in this case be performedpreferably with a microwave irradiation at outputs of between 200 and1500 W (2.45 GHz). The process gas or process gas mixture has in thiscase preferably a process pressure of between 0.3 and 1 mbar. By meansof the high-frequency feed 32, a frequency of, for example, 13.56 MHz isapplied and a plasma is generated. From the plasma generated, activatedparticles strike the starting material which has been charged into therotary drum 24. This effects a change in structure within the startingmaterial, that is of the charged polyolefin, in the form of theincorporation of polar groups (oxygen-containing and/ornitrogen-containing). These polar groups are both reactive groups(hydroxyl groups, carboxyl groups, primary and secondary amino groups)and non-reactive groups (carbonyl groups, tertiary amino groups). Acrosslinkage does not take place. By appropriately changing feeding ofdifferent process gases by means of the feed lines 30 and differentapplication of frequencies by means of the generator 28 or thehigh-frequency feed 32, influence can be brought to bear on differentcompositions of the starting materials.

In an actual example, LDPE (AL 22 FA) having a grain size of less than 1mm is charged into the rotary drum 24 as the starting material. Aprocess pressure of 0.75 mbar at a rotational speed of 8 revolutions perminute of the rotary drum 24 is set as the process parameter. The outputof the generator 28 is 600 W and that of the high-frequency feed 32 islikewise 600 W. A mixture of argon, oxygen and nitrogen is fed asprocess gases and the introduced starting material is plasma-treated forthe duration of 270 s. The generator 28 thereby generates a frequency of2.45 GHz and the high-frequency feed 32 generates a frequency of 13.56MHz. With the material obtained after completion of the productionprocess, a transparent, well-adhering coating can be applied to glass ata processing temperature of 160° C.

I claim:
 1. A process for producing a polymer that can be used as apowder coating material or a hot-melt adhesive, which comprisessubjecting at least one polyolefin to a low-temperature plasma treatmentat frequencies changing during the treatment in a frequency range offrom 30 kHz to 10 Ghz.
 2. The process of claim 1, wherein the plasmatreatment is carried out in a frequency range of from 13.56 MHz to 2.45GHz.
 3. The process of claim 1 or 2, wherein the plasma treatment iscarried out with combinations of varyingly high, changing frequencies.4. The process of claim 1, wherein the polyolefin is selected from thegroup consisting of polyethylene and polypropylene.
 5. The process ofclaim 1, wherein the plasma treatment is carried out under an inert gas.6. The process of claim 5, wherein the inert gas is selected from thegroup consisting of helium and argon.
 7. The process of claim 1, whereinthe plasma treatment is carried out under a reaction gas.
 8. The processof claim 7, wherein the reaction gas is selected from the groupconsisting of oxygen and nitrogen.
 9. The process of claim 1, whereinthe plasma treatment is carried out successively under an inert-gasplasma and at least one reaction-gas plasma is carried out successivelyunder an inert-gas plasma and a reaction-gas mixture plasma, or under amixture of at least one inert gas and at least one reaction gas.
 10. Theprocess of claim 1, wherein the plasma treatment is performed at aprocess pressure of from 0.3 mbar to 1 mbar.
 11. The process of claim 1,wherein the plasma treatment has a duration of 15 s to 600 s.
 12. Theprocess of claim 1, wherein the polyolefin is subjected to the plasmatreatment as a powder or as granules having a grain size of less than 1mm.
 13. The process of claim 1, wherein the polyolefin is moved duringthe plasma treatment.
 14. The process of claim 13, wherein thepolyolefin is agitated in a rotating drum during the plasma treatment.15. The process of claim 14, wherein the drum is rotated at a speed of 4to 20 revolutions per minute.
 16. The process of claim 1, wherein thepolyolefin is a material selected from the group consisting of virginpolyolefin material, recycled polyolefin material, and mixtures thereof.17. The process of claim 1, wherein at least one additive is added tothe polyolefin before the plasma treatment.
 18. The process of claim 17,wherein the additive is a stabilizer.
 19. Powder coating materialcomprising the polymer produced by the process of claim
 1. 20. Ahot-melt adhesive, comprising the polymer produced by the process ofclaim
 1. 21. A method for applying a protective coating to a materialwhich comprises coating the material with a polyolefin treated accordingto claim 1 wherein the material has a melting temperature lying above aprocessing temperature of the treated polyolefin.
 22. A method forbonding together materials which comprises adhesively bonding thematerials together with a polyolefin treated according to claim 1wherein the materials have a melting temperature lying above aprocessing temperature of the treated polyolefin.
 23. The method ofclaim 21 or 22, wherein the material is selected from the groupconsisting of glass, ceramic, steel, aluminum, wood, paper, andpolymers.
 24. The method of claim 21, wherein the material is precoated.25. The method of claim 21, wherein the material coated is theunderflooring of an automobile.
 26. The method of claim 21, wherein thematerial coated is a ship's hull.